Retroreflective conspicuity devices have been developed for use to increase safety and visibility especially during periods of reduced visibility. Generally, the problems related to attaching retroreflective conspicuity sheeting to rigid substrates have been solved. However, difficulties are encountered when it is desirable to attach retroreflective markings to a polymeric coated fabric material. Retroreflective conspicuity markings must be attachable to a flexible substrate, such as fabric material, without interfering with the life and function of the substrate.
Articles which use flexible fabric materials, such as a trailer tarpaulin or a roll-up sign, will typically have a life span up to about ten years. Flexible vehicles covers are particularly convenient, permitting the operator of the vehicle to gain access to the trailers quickly and conveniently, and to allow the trailer compartment to maintain reasonable weatherproofing abilities. The vehicle operator may open and close a cover numerous times each day, therefore the cover should be flexible but strong.
The vehicle cover must withstand harsh weather conditions as well as the mechanical demands placed on it by the operator. The covers encounter extremes in temperature, chemical challenges from atmospheric pollution and road salt, and photo-reaction involving infrared, visible and ultraviolet radiation from sunlight. A retroreflective cover must remain flexible and weatherproof throughout the expected life span.
Flexible fabric materials are typically fabrics manufactured from polyester, nylon or cotton. The fabric is usually coated with a suitable polymer, with the most useful being highly plasticized polyvinyl chloride (PVC).
Highly plasticized PVC is durable and convenient to work with. Highly plasticized PVC is normally attachable to itself or some other suitable polymers with the use of heat or radio frequency welding. Large fabric materials coated with PVC are manufactured by welding smaller panels together. Torn or damaged PVC coated fabric materials are often repairable while still on the vehicle. However, problems are encountered when attempting to use adhesives with PVCs due to the plasticizers which migrate from the PVC into the adhesive. This softens the adhesive and causes loss of its cohesive strength. Another problem relates to mechanical attachment, such as sewing, of materials to PVC flexible covers. This form of attachment often interferes with the waterproofing characteristic of a polymeric coated fabric material.
Other means of attaching PVC coated flexible fabrics include use of thermal and radio frequency energy. A thermal fusion technique, using heat for example from a source such as a hot air gun, increases the thermal kinetic motion of all of the atoms in the polymer chains. When the temperature of the polymer is increased to the melt temperature, the polymer is able to flow adequately to form a bond. For thermoplastic polymers, melting occurs at a temperature below the temperature at which degradation occurs. For suitable thermal fusion to occur, the polymers to be fused should have similar melting temperatures. An example of melting temperature compatibility is highly plasticized PVC and polyurethane. An example of incompatibility is highly plasticized PVC and polycarbonate, because of the substantially higher melting temperature for polycarbonate.
Radio frequency (RF) welding is an alternative to thermal fusion. RF welding accomplishes fusion through the presence of polymer polar groups converting the radio frequency energy into kinetic motion which heats the polymer. When a radio frequency field is applied to a thermoplastic polymer with polar groups, the ability of the polar groups to switch orientation in phase with the radio frequency will determine the degree to which RF energy is absorbed and converted to kinetic motion of the polar group. This kinetic energy is conducted as heat to the entire polymer molecule. If enough RF energy is applied, the polymer will heat sufficiently to melt. A useful measure in determining the degree to which a polymer will absorb energy from an alternating field is the relation of the polymer's dielectric constant and the dielectric dissipation factor known as the loss factor and is given by the following relationship: EQU N=5.55.times.10.sup.-13 (.function.)(.Fourier..sup.2)(K)(tan.delta.); eq. 1
where N is the electric loss in watts/cm.sup.3 -sec, .function. is frequency in Hertzisec, .Fourier. is field strength in volts/cm, K is the dielectric constant, and .delta. is the loss angle (tan.delta. is the dissipation factor).
This dissipation factor is the ratio of the in-phase to out of phase power. If the polar groups in a thermoplastic polymer have a relative inability to switch orientations in the RF field, this results in a phase lag. This phase lag is known as the loss factor. The higher the dissipation factor, the greater the amount of heat a RF field will generate. Studies with thermoplastic polymers and radio frequency welding have demonstrated that thermoplastic polymers with dissipation factors of approximately 0.065 or higher will form useful welds. For example, PVC has a dissipation factor of approximately 0.09 to 0.10 at 1 MHz, nylon caprolactam has a dissipation factor of 0.06 to 0.09 and polycarbonate has a dissipation factor of only 0.01. The respective dielectric constants for these three compounds are 3.5, 6.4, and 2.96 at 1 MHz.
Polyethylene, polystyrene, and polycarbonate have very low dissipation factors and in practical use have poor radio frequency welding capability. The polyvinyl chlorides, polyurethanes, nylon, and polyesters have reasonably high dissipation factors and have been found in practical use to form very functional RF welds. Reference is made to the article "RF Welding of PVC and Other Thermoplastic Compounds" by J. Leighton, T. Brantley, and E. Szabo in ANTEC 1992, pps. 724-728. These authors did not attempt to weld polycarbonate to the other polymers because of the understanding in the art that a useful weld, using RF energy, would always fail to form.
Only those polar groups within the RF field will be put into motion. The convenience of RF welding is realized by this controlled heating of only the molecules within the RF field. The need for thermal insulation is obviated by the use of RF welding.
PCT Application WO 93/10985 published Jun. 10, 1993, discloses attaching PVC retroreflective articles to a tarpaulin cloth coated with PVC using RF welding. This combination was then hot air fused to a tarpaulin vehicle cover also coated with PVC. To thermally weld the PVC coated cloth to the PVC coated tarpaulin cover, the two surfaces are heated to approximately 400 to 600.degree. C. and the surfaces then pressed together to accomplish the hot air fusion. The purpose of the intermediate tarpaulin cloth attachment was to provide thermal insulation between the hot air and the retroreflective article attached to the tarpaulin cloth to prevent thermal melting, loss of retroreflection and destruction of the retroreflective article.
Cube corner retroreflective articles constructed from PVC have relatively low coefficients of retroreflectivity, generally in the region of approximately 250 candelas per lux per square meter or less. A retroreflective flexible fabric material using high brightness flexible polymer prismatic retrororeflective elements that is relatively simple to attach to the flexible fabric would be desirable.